Targeting disease-causing gene sequences was first suggested more than thirty years ago (Belikova et al., Tet. Lett., 1967, 37, 3557-3562), and antisense activity was demonstrated in cell culture more than a decade later (Zamecnik et al., Proc. Natl. Acad. Sci. U.S.A., 1978, 75, 280-284). One advantage of antisense technology in the treatment of a disease or condition that stems from a disease-causing gene is that it is a direct genetic approach that has the ability to modulate (increase or decrease) the expression of specific disease-causing genes. Another advantage is that validation of a therapeutic target using antisense compounds results in direct and immediate discovery of the drug candidate; the antisense compound is the potential therapeutic agent.
Generally, the principle behind antisense technology is that an antisense compound hybridizes to a target nucleic acid and modulates gene expression activities or function, such as transcription or translation. The modulation of gene expression can be achieved by, for example, target degradation or occupancy-based inhibition. An example of modulation of RNA target function by degradation is RNase H-based degradation of the target RNA upon hybridization with a DNA-like antisense compound. Another example of modulation of gene expression by target degradation is RNA interference (RNAi). RNAi generally refers to antisense-mediated gene silencing involving the introduction of dsRNA leading to the sequence-specific reduction of targeted endogenous mRNA levels. Regardless of the specific mechanism, this sequence-specificity makes antisense compounds extremely attractive as tools for target validation and gene functionalization, as well as therapeutics to selectively modulate the expression of genes involved in the pathogenesis of malignancies and other diseases.
Antisense technology is an effective means for reducing the expression of one or more specific gene products and can therefore prove to be uniquely useful in a number of therapeutic, diagnostic, and research applications. Chemically modified nucleosides are routinely used for incorporation into antisense compounds to enhance one or more properties, such as nuclease resistance, pharmacokinetics or affinity for a target RNA. In 1998, the antisense compound, Vitravene® (fomivirsen; developed by Isis Pharmaceuticals Inc., Carlsbad, Calif.) was the first antisense drug to achieve marketing clearance from the U.S. Food and Drug Administration (FDA), and is currently a treatment of cytomegalovirus (CMV)-induced retinitis in AIDS patients.
New chemical modifications have improved the potency and efficacy of antisense compounds, uncovering the potential for oral delivery as well as enhancing subcutaneous administration, decreasing potential for side effects, and leading to improvements in patient convenience. Chemical modifications increasing potency of antisense compounds allow administration of lower doses, which reduces the potential for toxicity, as well as decreasing overall cost of therapy. Modifications increasing the resistance to degradation result in slower clearance from the body, allowing for less frequent dosing. Different types of chemical modifications can be combined in one compound to further optimize the compound's efficacy.
The synthesis of 5′-substituted DNA and RNA derivatives and their incorporation into oligomeric compounds has been reported in the literature (see for example: Saha et al., J. Org. Chem., 1995, 60, 788-789; Wang et al., Bioorganic & Medicinal Chemistry Letters, 1999, 9, 885-890; and Mikhailov et al., Nucleosides & Nucleotides, 1991, 10(1-3), 339-343) and Leonid et al., 1995, 14(3-5), 901-905).
The synthesis of 2′-amino and 2′-thio bicyclic nucleosides has been reported in the literature (see for example: International Application PCT/DK98/00393, filed Sep. 14, 1998, and published as WO 99/14226 on Mar. 25, 1999; International Application PCT/DK2004/000097, filed Feb. 10, 2004, and published as WO 2004/069992 on Aug. 19, 2004; Singh et al., Journal of Organic Chemistry, 1998, 63(18), 6078-6079; Pedersen et al., Synthesis, 2004, 4, 578-582; U.S. Application 20040014959, published Jan. 22, 2004; and U.S. Application 2004241717, published Dec. 2, 2004).
The synthesis of 2′-amino and 2′-thio bicyclic nucleosides and their incorporation into oligomeric compounds has been reported in the literature. Selected oligos have been looked at for evaluation of Tm, in vitro activity and in vivo activity (see for example: Kumar et al., Bioorganic & Medicinal Chemistry Letters, 1998, 8(16), 2219-2222; and Fluiter et al., ChemBioChem, 2005, 6, 1-6).